This invention relates in general to wood trusses and more particularly to wood trusses which are hung from their top chords.
Wood trusses find widespread use in the construction industry, primarily as a principal component of roofs and of floor systems. Irrespective of its location, the typical truss has wood top and bottom chords and usually a series of wood webs that are extended between the top and bottom chords, often oriented at oblique angles to such chords. The joints between the webs and chords are secured with truss connector plates, the typical plate being nothing more than a rectangular piece of steel having sharp projections extended from one of its faces. With a plate spanning a joint, the projections are driven into and embed within the lumber at the joint, thus firmly attaching the lumber and securing the joint.
So-called top hung trusses, that is trusses which are supported at the ends of their top chords, present a special problem, because the entire weight of the truss as well as the load that is carried by the truss is supported beyond the last webs of the truss, thus imparting significant shear forces and moments in these regions of the truss. At each end of a typical top hung truss, the top chord projects beyond the last web which intersects with it as well as beyond the connector plate which joins that web to the top chord (FIG. 1). While the load on the truss is along most of the truss distributed through the chords and webs, the load beyond the last web-chord joint in the top chord is carried entirely by the chord extension, that is by the portion of the top chord that projects beyond the endmost joint and rests on the bearing surface. This imparts shear and moment stresses to the chord extension.
The shear force and the moment tend to split the lumber perpendicular to the direction in which the shear force is applied. In this regard, wood is quite strong in the direction of its grain in tension and compression, but it is quite weak in shear. The shear forces and moment tend to cause the wood within the extension of the top chord to pull apart and split along the grain (FIG. 1).
Furthermore, the axial forces transmitted by the top chord and the last webs do not intersect near the center of the connector plate as is desirable, but instead intersect near the outside upper corner of that plate. This produces eccentricity in the joint or more accurately in the connector plate, requiring the plate to resist a large induced or secondary moment (FIG. 1).
Usually the bearing for a top hung truss is some sort of wood sill plate and when a truss is so supported, the load tends to concentrate along the inner edge of the sill plate, this to a large measure being the result of slight bending in the chord extension due to the moment caused by the offset of the load from the last connector plate. In any event, the uneven distribution of the load tends to crush the top chord extension in the region of the inner edge of the sill and if the sill is wood, it is likewise crushed along its inner edge. As a consequence, a certain amount of rotation occurs along the inner edge of the sill plate, and this rotation imparts a deflection to the truss itself.
Close tolerances are difficult to hold on large construction projects, and the typical 1/2 inch tolerance that is left as a working clearance beyond the endmost web can vary. The greater the clearance, the more pronounced the foregoing problems become.